Immediately following its discovery in 1946, nuclear magnetic resonance (Nuclear Magnetic Resonance, NMR) phenomenon found application in fields such as physics, chemistry, material science, life science and medicine. And the application of nuclear magnetic resonance in petroleum and natural gas industry, pioneered by utilization in the field of reservoir petrophysics, was initiated in the 1950s. A nuclear magnetic resonance logging instrument can perform stratum information detection around a wellbore by employing the principle of nuclear magnetic resonance, and is thus provided with unique capacity for qualitative identification and quantitative evaluation of reservoir fluid.
Probe is one of the important parts of a nuclear magnetic resonance logging instrument, and its structure determines key performances, such as measurement mode, nuclear magnetic resonance region and nuclear magnetic resonance signal intensity of the instrument. A nuclear magnetic resonance logging instrument probe mainly includes a magnet and an antenna, where the magnet can form a static magnetic field for polarizing a self-spinning hydrogen proton, while the antenna can transmit a radio frequency field for turning the self-spinning hydrogen proton around. And removing the radio frequency field prompts the self-spinning hydrogen proton to precess along the static magnetic field, resulting in nuclear magnetic resonance induction signals, detection of which makes it possible to analyze the formation condition.
In the prior art, a nuclear magnetic resonance logging instrument probe usually adopts a cylindrical magnet, in which the circular faces at two sides of the magnet serve as an N pole and an S pole, respectively, and closed magnetic induction lines directed from the N pole to the S pole form magnetic field distribution; an antenna encircles the magnet, allowing excitation of a polarized formation region around the wellbore in 360 degrees, thus making the region around the wellbore free of detection blind area, and enabling multi-frequency multi-section measurement, but the measured signal can only be an average signal of the signals in 360 degrees stratum. Therefore, the nuclear magnetic resonance logging instrument probe in the prior art can only conduct signal detection in a radial depth dimension and an axial depth dimension, and is incapable of carrying out signal detection of multidirectional sensitive region in the circumferential direction.